President Vladimir Putin on Friday said Russia's takeover of Crimea showed off its military prowess, as his defence minister reported that the Russian flag was now flying over all military sites on the Black Sea peninsula.

"The recent events in Crimea were a serious test," Putin was shown on state television as saying in an echoing and glided Kremlin hall.

"They demonstrated both the completely new capabilities of our Armed Forces and the high morale of the personnel."

A deputy head of the Federal Security Service, Alexander Malevany, told Putin at the meeting active measures were being taken to counter what he called growing Western efforts to weaken the Russian state and curb Moscow's influence in its post-Soviet backyard.

The United States and NATO have voiced alarm over what they say are thousands of Russian troops massed near its western border with Ukraine. Putin has reserved the right to send troops into Ukraine, which is home to a large population of Russian-speakers in the east.

Putin praised Russian troops for "avoiding bloodshed" in Crimea, whose largely ethnic Russian citizens overwhelmingly voted to join Russia in a March 16 referendum dismissed in the West as illegal.

Speaking to Ukrainian servicemen who chose to swap sides and swear allegiance to Russia, Putin hinted they would be well rewarded by pointing out that Russian servicemen earned some four times more than their Ukrainian counterparts.

Ukraine's former navy chief, Rear Admiral Denys Berezovsky, has been handed the deputy command of Russia's Black Sea Fleet after his defection.

"The change in state symbols on all ships and in all divisions that have sided with the Russian army has been completed," Shoigu told Putin.

He said warships, war planes and other hardware seized from troops loyal to Kiev will be returned to Ukraine, which could help Russia avoid a potentially costly legal battle in international arbitration courts.

The Foreign Ministry also said Moscow would move to annul its lease agreement with Kiev that allowed it to base its Black Sea Fleet on the peninsula.

THREATS FROM THE WEST

Despite signs that tensions with the West may be cooling as a status quo takes shape in Crimea, Malevany warned Putin at Friday's meeting that Moscow faces growing threats from the United States and its allies, who are trying to weaken Russia's influence on Ukraine.

"There has been a sharp increase in external threats to the state," he said. "The lawful desire of the peoples of Crimea and eastern Ukrainian regions is causing hysteria in the United States and its allies."

U.S. President Barack Obama said on Friday that Russia's troop buildup near Ukraine's eastern border may be more than just an effort to intimidate Ukraine and urged Moscow to pull forces back to ease tension.

Putin has received permission from parliament to send the armed forces into Ukraine if necessary, raising concerns he could cite alleged threats to Russian-speakers in eastern regions as grounds for intervention there.

The Foreign Ministry has fuelled such concerns with near daily reports that ethnic Russians were being targeted.

"Total ignorance of the interests of Russian-speaking residents of Ukraine is now nothing new but rather a permanent reality," it said on Friday.

It called on international organisations to pay attention to what it called severe violations of the rights of minorities by Ukraine's pro-Western interim government.